1. Field of the Invention
The present invention relates to organic electroluminescent (EL) devices, and more specifically, novel blue emitting organic EL devices with excellent performance characteristics. The organic EL devices of the present invention contain charge transport/luminescent materials comprised of a new class of organic compounds that may enable the aforementioned characteristics. These devices can be selected for use in flat-panel emissive display technologies, including TV screens, computer screens, and the like.
2. Discussion of the Related Art
Organic light emitting devices (OLEDs) represent a promising technology for display applications. For the production of a full-color EL display, it is desirable to have organic EL devices that are capable of providing uniform luminescence, saturated color in blue, green and red, and low driving voltages. A typical organic EL device can be comprised of a layer of an organic luminescent material conductively sandwiched between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals.
EL devices function on the principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. Typically, for visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers for blue, green and red.
Several prior art organic EL devices have been constructed from a laminate of an organic luminescent material and electrodes of opposite polarity. These devices typically include a single crystal material, such as single crystal anthracene. However, these devices usually require excitation voltages on the order of 100 volts or greater.
Organic EL devices with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Another alternative device configuration is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode.
Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent dopant material.
In U.S. Pat. No. 4,539,507, the disclosure of which is fully incorporated herein by reference, there is disclosed an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p-tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4′-bis(5,7-di-tert-pentyl-2-benzoxyzolyl)stilben, and an indium cathode. In U.S. Pat. No. 6,229,012, the disclosure of which is also fully incorporated herein by reference, there are illustrated devices with certain carbazoles.
U.S. Pat. No. 4,720,432, the disclosure of which is fully incorporated herein by reference, discloses an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport.
U.S. Pat. No. 4,769,292, the disclosure of which is fully incorporated herein by reference, discloses an EL device employing a luminescent zone comprised of an organic host material capable of sustaining hole-electron recombination and a fluorescent dye material capable of emitting light in response to energy released by hole-electron recombination. One host material disclosed in the '292 patent is an aluminum complex of 8-hydroxyquinoline, and more specifically, tris(8-hydroxyquinolinate)aluminum.
U.S. Pat. No. 5,409,783, the disclosure of which is fully incorporated herein by reference, discloses a red-emitting organic EL device containing a dopant of a tris(8-hydroxyquinolinate)aluminum with a red fluorescent dye. Further, blue-emitting organic EL devices are illustrated in, for example, U.S. Pat. Nos. 5,151,629 and 5,516,577, the disclosures of which are fully incorporated herein by reference.
For efficient light emission in an OLED a guest-host system is often used which helps with charge distribution within the OLED and combats the formation of exiplexes in the emitting material. A good guest-host system is one in which electrons and holes are transported through the guest-host matrix leading to complete charge recombination on the guest. It is desirable to have a band-gap of the guest that falls within the band-gap of the host.
While progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of a number of devices are still below expectations in several ways. While the creation of efficient and stable green and red fluorescent materials has advanced in the past years, the same is not true for blue emitting materials. Forming a good guest-host system for blue emitting organic EL devices is very difficult. This is due to the fact that blue light-emitting materials have a low affinity for the electron from the cathode in OLED devices. This has caused the performance of blue emitters to be inferior to that of green or red emitters. Accordingly, there is a need to develop stable blue emitters with high color purity.